1. Field of the Invention
The present invention relates to image sensors, and more particularly relates to CMOS image sensors and methods of operating and fabricating the same.
2. Description of the Related Art
The rapid advancement of digital camera technology and a resulting increase of their popularity has made high performance digital cameras in high demand among consumer electronics. The basic components of a digital camera that determine its performance level are an optical lens and an image sensor. The image sensor functions to transform light applied thereto (through the optical lens) into an electronic signal representing an image.
A typical image sensor is composed of a pixel array in which a plurality of pixels are arranged in a two dimensional matrix. Each pixel includes a light detection unit (or a photodetector), and a transmission and signal output (readout) unit. Image sensors may be normally classified into two types: the charge coupled device image sensors (hereinafter, referred to as CCD image sensors); and the complementary metal-oxide-semiconductor image sensors (hereinafter, referred to as CMOS image sensors). The CCD image sensor employs MOS (metal oxide semiconductor, field effect transistor) capacitors for transferring and outputting signals and charge carriers are stored in a capacitor are transferred to an adjacent capacitor, by a potential difference between the capacitors. By comparison, the CMOS image sensor utilizes a switching scheme sequentially detecting output signals of MOS transistors that are formed in the pixels thereof.
Thus, the CCD image sensors are able to take better images than CMOS image sensors because of having less noise therein, but have disadvantages of higher product costs and larger power consumption. In other words, the CMOS image sensors have the advantages of lower power operation, (low power consumption), compatibility with integrated or operatively coupled CMOS circuits, random accessing of image data, reduced cost (e.g., by using standard CMOS techniques), and so forth. Thus, the CMOS image sensors are more widely used for various applications such as digital cameras, smart phones, PDAs, notebook computers, security cameras, barcode sensors, high definition televisions, high resolution cameras, toys, and so on.
FIG. 1A is an equivalent circuit diagram illustrating the pixel structure of a conventional CMOS image sensor including a photo-receiving device and four transistors (hereinafter, referred to as ‘four-transistor pixel structure’), and FIG. 1B is a timing diagram illustrating an operation of the CMOS image sensor of FIG. 1A.
Referring to FIG. 1A, the image sensor with four-transistor pixel structure is composed of four transistors, (i.e., a transfer transistor 13, a reset transistor 15, a drive transistor 17, a selection transistor 19), and a photo-receiving device 11.
The typical operation of the four-transistor pixel structure is as follows. Referring to FIG. 1B, a selection voltage φSG is applied to the (selection) gate of the selection transistor 19 during a signal output period Td, turning the selection transistor 19 ON. After the selection transistor 19 is turned ON, a reset voltage φRG is applied to the (reset) gate RG of the reset transistor 15, by which the reset transistor 15 is turned ON to reset a floating diffusion node 14 to a power supply voltage level VDD approximately. Thereby, the pixel is reset. Then, the supply voltage level VDD is applied to the (drive) gate DG of the drive transistor 17 as a drive voltage φDG, so that a reference voltage Vref is provided to an output node Vout within a first signal output period Td1.
After resetting the pixel, when light is incident upon the photo-receiving device 11, electron-hole pairs (EHP) are generated proportionally in response to the incident light. And then, if a transfer voltage φTG is applied to the transfer gate TG, the potential barrier (resistance) between the photo-receiving device 11 and the floating diffusion node 14 becomes lower (allowing a transfer of signal charges from the photo-receiving device 11 to the floating diffusion node 14). Thereby, the potential at the floating diffusion node 14 varies in proportion with the amount of signal charges transferred thereto. Thus, the drive voltage φDG applied to the drive gate DG drops down under the initial (supply) voltage VDD, and a (pixel) signal data voltage Vpix appears at an output node Vout within a second signal output period Td2. An image signal is output as a value arising from a difference value Vsig between the reference voltage Vref and the signal data voltage Vpix.
As such, it is very important to entirely transfer the signal charges, which are generated at the photo-receiving device 11, to the floating diffusion node 14 through the transfer gate TG. If the signal charges generated remain in the photo-receiving device 11 without being wholly transferred to the floating diffusion node 14, the remaining signal charges causes the phenomenon of “image lagging” that leaves afterimages in the next frame, resulting in degradation of picture quality in the image sensor.